Warming blankets incorporating electrically activated heating elements are well known. It is also known to provide warming blankets that incorporate sensor wires in combination with heating elements so as to monitor the level of heat generation. In the construction of warming blankets, it is well known to use wrapped wire constructions in which complementary heating and sensing wires are wrapped around a structural core such as an elongate polymeric fiber or the like. In some prior known constructions, the heating and sensor wires have been disposed within a common insulated covering forming a unitary elongate structure. The elongate structure housing the heating and sensor wires is then threaded in a desired pattern through channels at the interior of the blanket. The wires may be wrapped concentrically with an insulating sleeve between the wires such as disclosed in U.S. Pat. No. 6,153,856 or in a coaxial arrangement such as disclosed in U.S. Pat. No. 5,861,610 to Weiss. It is also known to use double wrapped wires with either a melt down layer or temperature coefficient material between the two wires such as described in U.S. Pat. No. 4,742,212 to Ishii.
In operation of prior heating blanket constructions, an electrical current is passed through the heating and sensor wires causing the heating wire to increase in temperature. The electrical properties of the sensor wire change with temperature in a predetermined manner. Thus, by monitoring the applied current and voltage across the sensor wire, the temperature of the sensor wire can be determined and the current to the heating wire can be increased or decreased so as to raise or lower the temperature of the blanket as desired.
As will be appreciated, in order for a feedback control system to be operable, the sensor wires must be arranged in a complete circuit. In the past, heating and sensor wires have been threaded through interior spaces within the blanket. While such structures may perform well, they may be difficult to manufacture and are not readily susceptible to continuous manufacturing processes.